Generally speaking, electromechanical machines are used in many applications where it is necessary to convert electrical energy to rotational mechanical energy. The speed of the rotation is governed by the parameters of the electromechanical machine. Parameters governing the speed include whether the machine is AC or DC, the voltage level of the input, the design of the machine itself, and the frequency of the input voltage. The last of these parameters, the frequency of the input voltage, can be varied to control the speed of the machine to a high degree of accuracy.
These variable speed electromechanical drive systems are used in numerous industries, including manufacturing, transportation, and power generation. Recent innovations in high power semi-conductor technologies have led to an exponential growth of the applications of variable speed electromechanical drive systems. With the of variable speed drives, however, new problems have arisen due to the fast switching speeds of the power electronics of the drive system.
One such problem is the fluting of motor bearings caused by stray currents generated in the field of the motor that are not properly insulated from the shaft of the electromechanical machine. Fluting is a type of electrical discharge machining, which results in a corrugated pattern often found on the surface of the inner or outer race of a bearing. This type of damage reduces the life of bearings that should last from six to ten years to as little as four months. It is estimated that millions of dollars are lost to this type of bearing damage because of the increased maintenance necessary when damaged shafts and other components must be replaced.
The present invention is directed to a method and arrangement that are designed to reduce the stray currents passing through motor bearings to thereby minimize the damage caused by the problem of fluting.